A great attention is directed recently to a spread spectrum communication system (hereinafter called "SSC system") as a new communication system. A receiver arrangement of such an SSC system must include a correlation function.
Since a SAW convolver has a correlation function and is operative as a programmable matched filter, it is one of the most important devices of an SSC.
The following three arrangements are proposed as a SAW convolver.
A separate-medium arrangement includes a semiconductor such as silicon and a piezoelectric film such as lithium niobate coupled to each other via a small gap.
An elastic arrangement includes input comb-shaped electrodes and an output gate electrode formed on a piezoelectric film such as lithium niobate to use an elastic non-linearity of the piezoelectric film.
A multi-layer arrangement includes a semiconductor such as silicon substrate on which a piezoelectric film such as zinc oxide is grown by a sputtering.
These three arrangements includes input electrodes and an output electrode. FIG. 18 is a plan view of an elastic arrangement, and FIG. 19 is a side elevation thereof. In FIGS. 18 and 19, reference numeral 1 refers to a lithium niobate or other piezoelectric substrate, 2 and 3 to input electrodes in the form of an aluminum or other metal film, 4 to an output electrode in the form of an aluminum or other rectangular metal film, and 8 and 9 to absorbers for attenuating undesired surface acoustic waves.
In FIGS. 18 and 19, an electric signal applied to terminals 5 and 6 of the input electrodes 2 and 3 is converted to SAW's which propagate from the electrodes 2 and 3 along the surface of the piezoelectric substrate 1. A SAW generated by the input electrode 2 travels to the right and to the left. However, since any undesired SAW reflected back by an end portion and travelling to the left is absorbed by the absorber 8, the arrangement can prevent any SAW travelling back to the right. Similarly, any rightwardly travelling SAW among SAW's travelling right and left is absorbed by the absorber 9.
That is, as shown in FIG. 20, SAW S1 from the input electrode 2 and SAW S2 from the input electrode 3 are conjoined via a nonlinear interaction so that a convolution output electric signal is extracted from an output terminal 7.
However, as shown in FIG. 21, when SAW S1 travels rightwardly from the input electrode 2 to the other input electrode 3, a component thereof is reflected back by the input electrode 3 and travels again to the left as SAW component S11. Similarly, when SAW 2 travels leftwardly from the input electrode 3 to the other input electrode 2, a component thereof is reflected back by the input electrode 2 and travels again to the right as SAW component S22.
As explained above, undesired convolution signals derived from a relative function between SAW S1 and SAW S11 and an interaction between SAW S2 and SAW S22 are outputted in addition to desired convolution signals between SAW S1 and SAW S2. These reflections are caused mainly by a so called re-emission and an acoustic impedance discontinuity. The re-emission is a phenomenum that SAW S1 and SAW S2, after travelling to the opposite input electrodes and converted to electric signals, are converted again to SAW's. The acoustic impedance discontinuity is caused by presence or absence of metal at the input electrode portions.
Since a SAW convolver in general has a small number of pairs of electrodes, the re-emission is the most important reason of the reflections. The convolutions between S1 and S11 and between S2 and S22 are called "Self-Convolutions" because they are convolutions by signals derived from themselves.
Since these self-convolutions are spurious signals, they deteriorate the SAW convolver characteristic.
The aforegoing phenomenum is immaterial when the input electrodes are unidirectional transducers because reflected components S11 and S22 are suppressed. However, an SSC requires a wide-band unidirectional transducer in order to deal with wide band signals. Although various narrow-band unidirectional transducers are proposed heretofore, wide-band unidirectional transducers have complicated arrangements, and it is difficult to cover necessary bands of an SSC sufficiently. Therefore, self-convolutions are usually present as shown in FIG. 21.
In order to suppress the self-convolution, I. Yao proposes a double track arrangement shown in FIG. 22 in "High Performance Elastic Convolver With Parabolic Horns" in 1980 Ultrasonics Symposium Proceedings, I . . . , Pages 37 to 42.
In FIG. 22, reference numerals 10 and 11 denote one pair of input electrodes, whereas 12 and 13 denote the other pair of input electrodes. Reference numeral 14 and 15 designate output electrodes whose outputs are sent to a balance-unbalance converter 18 to subsequently extract a total convolution output through 19. When a signal is applied to an input terminal 16, SAW's travel to the right in parallel relationships along two tracks corresponding to the input electrodes 10 and 11 and reach the opposite input electrodes 12 and 13. These entering SAW's, however, are opposite in phase at the input electrodes 12 and 13, and their sum output is produced at a terminal 17. Therefore, no electric signal derived from the SAW's is detected at the terminal 17, and no re-emission phenomenum occurs. Beside this, reflected components caused by the discontinuity of the acoustic impedance is deleted by the balance-unbalance converter 18. As a result, the total self-convolution is largely suppressed.
However, as shown in FIG. 22, since two output electrodes 14 and 15 are disposed in a parallel relationship, this arrangement requires an area double the arrangement of FIGS. 11 through 14. This necessarily increases the material cost and the dimension. Further, the use of the balance-unbalance converter 18 also increases the manufacturing cost and the entire dimension.
Although the aforegoing explanation is directed to the elastic arrangement, the separate-medium arrangement and multi-layer arrangement also include the same problems.